Various types of apparatus for purifying gases to break down or neutralise the polluting substances are known. In one known type, also used in catalytic converters for the exhaust gases from internal combustion engines a thermal reactor is used in which the mass of the reactor is brought to a high temperature to achieve complete oxidation and/or thermal dissociation of the molecules of the polluting substances. For this operation it is first necessary to reduce the velocity of the gas flow drastically by passing it through a series of chambers or into a highly obstructive catalytic path.
This apparatus is in general very expensive both because of the energy required in operation and because of the equipment required for regulating and controlling it. Moreover it cannot be used when the gases to be purified contain molecules of heavy metals, chlorinated hydrocarbons, phosphorus, arsenic, boron, lead, zinc and like toxic substances.
Finally even for the substances for which this apparatus is effective, large quantities of carbon dioxide are released which is known to stratify at relatively low contents. A barrier or greenhouse is thus formed which means that heat accumulates causing harmful variations in local climate, for example in large cities.
Apparatus is also known for purifying gas by means of chemical reactions in which the pollutant is transferred to liquid or solid reagents. When these reagents become saturated or exhausted, however, they become corrosive and/or caustic whereby it is not easy to dispose of them ecologically.
Finally, purifying apparatus is known which is based on the reaction of molecules in a gas with molecules on the surface of a solid material, known as adsorption. In this, the gas is retained by the surfaces of grains of adsorbent material, for example activated carbon, in a so-called adsorption reactor. These reactors, however, have the disadvantage that the surfaces of the grains rapidly become clogged so that they have to be regenerated frequently. Hence, in general, this type of apparatus may be used solely with gases having low concentrations of substances to be removed. Moreover, in a plant which operates in a continuous cycle, the apparatus must be duplicated so that there is always one unit working while the other is being regenerated.